1. Field of the Invention
The present invention relates to a rear-focus zoom lens system with a variable power ratio of about 12 and a brightness, or F value, approximately in the range from 1.6 to 1.8 and, more particularly, to a compact-size high variable power zoom lens system having a large view angle, i.e., 65.degree. or greater. This zoom lens system is useful for a video camera or a still camera.
2. Description of the Related Art
As the size and weight of video cameras and still cameras are being reduced, there is a growing need for smaller and lighter lenses. In response to this need, a variety of systems have been proposed. Some of the systems use the conventional, and so-called front lens focus method, which performs focusing by moving the first lens group, the first lens group being the closest to an object and larger than the other lens groups. Some other systems use a so-called rear focus method, which performs focusing by moving a lens group other than the first lens group.
Generally, because a zoom lens system of the rear focus type moves a relatively small and light-weight lens group to perform focusing, that method reduces the required driving force and provides for a speedy focusing.
For example, the assignee of the present invention has proposed, in Japanese Patent Applications Laid-Open Nos. 62-24213 and 62-215225, a zoom lens system of the rear focus type which comprises: a stationary first lens group; a movable second lens group for varying the magnification; a stationary third lens group having a positive refractive power; and a movable fourth lens group having a positive refractive power. In that combination, the second lens group moves to vary magnifications and the fourth lens group moves to compensate for changes in the image field caused by magnification variations and also to perform focusing.
In order to reduce the size of the first lens group, in both a front lens focus type and a rear focus type, the focal length at the wide-angle end (which determines the size of the first lens group) typically is made as close to the telephoto end as possible. Therefore, in most lens systems of these types, the view angle at the wide-angle end is generally 50.degree. or so. A lens system having a view angle of the wide-angle end greater than 50.degree. inevitably has required a complicated lens system, which would not meet the need for a compact size.
To obtain a view angle of 65.degree. or greater in a video camera, either a wide angle converter lens or a wide angle attachment lens typically has been attached in front of the lens system, which is a troublesome operation.